1. Field of the Invention
The present invention relates to a level shift circuit that is provided with plural circuit sections that operate by different power supplies and which performs signal transmission between the circuit sections.
2. Description of the Related Art
Conventionally, in control chips and the like of a configuration that uses plural power supplies of different power supply voltages to cause circuits to operate, a level shift circuit has been used when performing signal transmission between the circuits that are operated by the different power supply voltages.
This type of level shift circuit operates normally in a state where electrical power of each of the power supply voltages is being supplied. However, when any of the supplies of electrical power is cut off, sometimes a through current flows and output becomes indefinite. When used for the purpose of operating by a battery, this through current lowers the life of the battery.
For example, in Japanese Patent Application Publication (JP-A) No. 2006-173889, as shown in FIG. 4, there is disclosed a level shift circuit 100 that is configured by an input circuit 101 that operates by a power supply voltage VDD1 and an output circuit 102 that operates by a power supply voltage VDD2. This level shift circuit 100 is provided with a NAND gate circuit 105, to which a control signal PD that is inputted from a control circuit (not shown) is inputted after being inverted by an inverter circuit 106 and which fixes the output level of the output circuit 102, and NOR gate circuits 103 and 104, to which an inverted signal of this control signal PD is inputted after being further inverted by an inverter circuit 107 and which fix the output level of the input circuit 101.
In the level shift circuit 100 that has been configured in this manner, when the power supply voltage VDD1 of the input circuit 101 has been cut off, the control circuit that causes the output circuit 102 to operate outputs an H level signal as the control signal PD.
Thus, in the output circuit 102, the control signal PD that has been inverted by the inverter circuit 106 is inputted to the NAND gate circuit 105 of the output circuit 102, and the output level of the output circuit 102 is fixed.
On the other hand, in the input circuit 101, the control signal PD that has been inverted by the inverter circuit 106 is further inverted by the inverter circuit 107 and is inputted to the NOR gate circuits 103 and 104 of the input circuit 101, and the output level of the input circuit 101 is fixed to an L level. As a result, N1 and N2 of the output circuit 102 becomes OFF.
By performing control in this manner, the level shift circuit 100 can cut off the path of a through current.
However, in the level shift circuit disclosed in JP-A No. 2006-173889, when supply of electrical power of the power supply voltage VDD2 is cut off in a state where electrical power of the power supply voltage VDD1 has been supplied, the output of the inverter circuit 107 in the output circuit 102 becomes indefinite. Moreover, the control signal PD is not transmitted to the input circuit 101, so a through current ends up flowing to the NOR gate circuits 103 and 104 of the input circuit 101.
Here, it is also possible to prevent a through current from flowing by first starting the power supply of the power supply voltage VDD2 when starting the power supplies and stopping the power supply voltage VDD2 last when stopping the power supplies. However, in an IC that is provided with plural circuit sections that are operated by different power supplies and that perform reciprocal signal exchanges therebetween, controlling the order of starting and stopping of the power supplies to prevent the creation of a through current path ends up becoming complicated.
The present invention provides a level shift circuit that can reliably cut off the path of a through current regardless of the state of the supply of power supplies to plural circuit sections that operate by different power supplies.